FIG. 1 illustrates a classic push-pull output stage for an operational amplifier. Pull-up transistor QP provides output current IL in the positive direction by pulling the potential of the output node VOUT toward the positive power supply rail VP, while pull-down transistor QN provides output current in the negative direction by pulling VOUT toward the negative supply rail VN. Diodes D1 and D2, which are typically realized as diode-connected transistors, reduce crossover distortion by causing the circuit to behave as a class AB amplifier with QP and QN each conducting a small bias current when the input signal VIN is zero. Diodes D1 and D2 also form a translinear loop with the base-emitter junctions of QP and QN to provide a well-defined, constant product characteristic where the product of IP and IN is proportional to the bias current IB.
Because the output transistors QP and QN are arranged in a common-collector (emitter follower) configuration, the potential of the output terminal VOUT must always be at least one base-emitter voltage (VBE) below the positive rail VP and one VBE above the negative rail VN. This may reduce the available output voltage swing to an unacceptable range, especially in systems with low power supply voltages.
To provide rail-to-rail operation, the output transistors may be arranged in a common-emitter configuration as shown in FIG. 2. Although this increases the output swing, it generally prevents the output transistors from forming part of a translinear loop. Therefore, replication transistors QRP and QRN may be used to generate currents IRP and IRN that replicate the currents through the output transistors. The replica currents may then be applied to a translinear loop in a drive circuit 6 which is used to drive the output transistors QP and QN.
U.S. Pat. No. 6,104,244, which is by the same inventor as the present disclosure, describes some improved rail-to-rail amplifier techniques in which one of the output transistors is arranged in a translinear loop, while a replica of the current from the other output transistor is applied to the loop in a manner that may maintain constant product operation.
FIG. 3 illustrates another approach to providing a rail-to-rail output in which one of the output transistors QN is arranged in a current minor configuration with another transistor QA. A drive circuit 8 provides separate drive signals to the output transistors QP and QN in response to the input signal VIN. A disadvantage of this configuration, as well as some of the other configurations above, is that the pull-up and pull-down portions of the circuits are driven asymmetrically. This lack of symmetry in sourcing and sinking current to a load, as well as other nonlinearities, may cause distortion in the output signal.